﴿ يَٰٓأَيُّهَا ٱلَّذِينَ ءَامَنُوٓا۟ إِذَا تَنَٰجَيْتُمْ فَلَا تَتَنَٰجَوْا۟ بِٱلْإِثْمِ وَٱلْعُدْوَٰنِ وَمَعْصِيَتِ ٱلرَّسُولِ وَتَنَٰجَوْا۟ بِٱلْبِرِّ وَٱلتَّقْوَىٰ ۖ وَٱتَّقُوا۟ ٱللَّهَ ٱلَّذِىٓ إِلَيْهِ تُحْشَرُونَ ﴾

“[Hence,] O you who have attained to faith, when you do hold secret confabulations, do not conspire with one another with a view to sinful doings, and aggressive conduct, and disobedience to the Apostle, but [rather] hold counsel in the cause of virtue and God-consciousness: and [always] remain conscious of God, unto whom you all shall be gathered.”

As a rule, secret whispering is an undesirable action. However, sometimes for some good reason, secret whispering has to be resorted to. In this case, the decisive factor is intention. If it is done with good intentions, it is legitimate but if it is done with evil intentions, it is sinful.